Detect change on the intersection of video and viewport.

media/base/media_observer.h

Index: media/base/media_observer.h
diff --git a/media/base/media_observer.h b/media/base/media_observer.h
index c3e29a1789f173d2cc62ad12845392cfaf1a70f7..20c660279be9f75e055ad9edbd65e08145e3cba1 100644
--- a/media/base/media_observer.h
+++ b/media/base/media_observer.h
@@ -20,6 +20,10 @@ class MEDIA_EXPORT MediaObserver {
   virtual void OnEnteredFullscreen() = 0;
   virtual void OnExitedFullscreen() = 0;
 
+  // Called when the media element starts/stops stably filling most of the
+  // viewport.
+  virtual void OnStablyFilledViewportChanged(bool is_mostly_filling_viewport) {}
+
   // Called when CDM is attached to the media element. The |cdm_context| is
   // only guaranteed to be valid in this call.
   virtual void OnSetCdm(CdmContext* cdm_context) = 0;